(1) Field of the Invention
The present invention relates to a camera for visual inspection to be used for optically inspecting presence of surface defects on an object having a complicated outer configuration.
(2) Related Art Statement
A conventional camera for visual inspection used for checking defects on a sheet shape body, such as a glass sheet or a steel sheet, comprises a linear image sensor having a plurality of sensor elements arranged linearly on an optic axis of the lens of the camera. However, as this kind of the camera for visual inspection is to convert the brightness of the surface of an object directly into an electric signal, it has an essential disadvantage in that the device can not identify the brightness variation due to presence of defect from the brightness variation caused from uneven illumination when the device is used for an object having a complicated rugged surface such as an insulator. In one method for improving the aforementioned disadvantage, a visual sensor system is possible to be considered, in which two linear image sensing devices are closely arranged on the focus plane of a camera in parallel direction to the plane and the influence of the uneven illumination is cancelled by applying operation to take difference between the output signals derived from the two linear image sensing devices.
However, a conventional linear image sensing devce is usually equipped with a transfer gate and a shift register around the light receiving portion so that it can not be arranged very closely less than a certain distance of an order of about 10 mm since the overall width of the device is already about an order of 10 mm. If the magnification of the lens is assumed as M, the device is to be arranged to compare the brightness on lines apart from 10.times.M mm each other on the surface of the object. Accordingly, it has been turned out that the measuring accuracy can not be improved over a certain value. Although the linear image sensing device is manufactured under a condition of very strict quality control, the light sensitivity may vary in a range of a few percent along the direction of array of the sensor elements. So there is a problem of deterioration of the inspection accuracy when two linear image sensing devices having different sensitivity characteristics are used.
Furthermore, for inspecting all the outer surface of an object having a complicated and rugged surface configuration such as an insulator, it is necessary to use a plurality of cameras as shown in FIG. 16. In this case, the required field of view for each one of the cameras varies greatly so that if cameras having a same view angle are used, the distance between the camera and the object must be varied for camera by camera and hence the magnification of the camera varies which results a problem of causing variation of the inspection accuracy. If the number of cameras are arranged at a certain constant distance from the object, there will be a case in which a portion of the surface other than the desired inspecting surface is included in the field of view of a camera as shown by the dotted lines in FIG. 16. It has been turned out that an accurate inspection is not possible in such case since some spurious noise waveforms just like the defect signals appear at both extremities of the field of view of the camera as can be seen from lines (A) to (C) in FIG. 17.